1. Field of the Invention
In self-igniting internal combustion engines, injection systems of many various designs are presently in use. Along with distributor injection pumps, reservoir-type injection systems with a high-pressure reservoir (common rail) are used as injection systems, as are unit injector systems (UIS) and pump-line-nozzle injection systems (UPS). Distributor injection pumps, unit injector systems (UIS) and pump-line-nozzle systems (UPS) are cam-driven injection systems, in which via a cam coupled in articulated fashion to the piston, a reciprocating motion is impressed upon a piston that dips into a pump work chamber. If magnet valves are used for controlling the injection event at the aforementioned cam-controlled injection components, then assurance must be provided that excessively long triggering times during which excessively high operating pressures arise, cannot occur.
2. Description of the Prior Art
European Patent Disclosure EP 0 178 427 B1 has an electrically controlled fuel injection pump for internal combustion engines as its subject. The electrically controlled fuel injection system can be used particularly with a Diesel engine. It includes at least one pump piston, which is driven with a constant stroke and defines a pump work chamber, and in the pumping stroke, the pump piston pumps the fuel, delivered at inlet pressure to this pump work chamber by a feed pump, to an injection nozzle at injection pressure. The pumping of fuel continues until a valve member of an overflow valve, actuated by an electrical actuator, blocks the flow of the fuel that otherwise spills over from the pump work chamber to a low-pressure chamber via a overflow conduit. The fuel injection pump further includes structural spaces in the overflow valve that receive a core and a conductor coil as well as an armature, and also includes a pressure chamber surrounding the valve member in the region of one end portion. A guide shaft is guided on the valve member in a guide bore and is prestressed by a compression spring. At the transition from the pressure chamber to a first portion of the overflow conduit that communicates with the low-pressure chamber, there is a conical valve seat that can be closed by a conical closing face. The overflow valve is inserted between this first portion and a second portion of the overflow conduit that connects the pressure chamber permanently to the pump work chamber. The valve member of the overflow valve opens inward, toward the pressure chamber that can be put at injection pressure. The cone angle α of the radial conical closing face is larger than the cone angle β of the associated valve seat, which widens conically toward the pressure chamber; with an adjacent cylindrical jacket face, on the end portion of the valve member, the closing face forms a precisely defined sealing edge. The conical valve seat has a narrow, hydraulically operative seat face that in the closed state of the overflow valve is closed by the closing face of the valve member and that is defined on the inside by the diameter of a flow opening in the first portion of the overflow conduit. The seat angle difference α-β of the two cone angles α, β is very small. The overflow valve is a needle valve that is open when without current, and whose valve member is embodied as a valve needle that is prestressed in the opening direction by the compression spring. On the face end, this valve needle has a needle tip, carrying the closing face, on its end portion remote from the actuator. The end portion is connected to the actuator via the guide shaft that is guided with narrow play in the guide bore. Between the guide shaft and the jacket face of the needle tip adjacent to the closing face, there is an annular-groove like constriction that enlarges the volume of the pressure chamber. The structural spaces that receive the core with the conductor coil and the armature communicate with the low-pressure chamber via a relief bore. With a rotationally symmetrical extension, which on its face end has a first spring abutment for a compression spring, the needle tip defined radially by the sealing edge protrudes into the first portion of the overflow conduit, communicating with the low-pressure chamber. A second spring abutment for the compression spring is inserted into the first portion of the overflow conduit, and the narrow, hydraulically operative seat face covered by the closing face at the needle tip of the valve member is only a few tenths of a millimeter wide. The diameter of the sealing edge at the end portion of the valve member is equal to the guidance diameter of the guide shaft, or is only slightly smaller than that diameter.
In the known embodiment, the switching magnet valves include a pressure step, which is defined by a diameter difference between the valve needle guide and a seat. By exerting a hydraulic force on the pressure step, the opening motion of the injection valve member, embodied as a nozzle needle, is reinforced. The manufacture of the pressure step in switching magnet valves involves tolerances. Completed components can vary sometimes considerably, within specified production tolerances, from one example to another. The result is variation in terms of the tolerance-dependent functional parameters from one component to another. This tolerance-dependent deviation of the functional parameters is thus also reflected in a deviation in the maximum operating pressure at which a control valve will open. Under some circumstances, this can happen at various relatively large values, so that a reliable, adequate protective function can be achieved only with difficulty.